Remove nightly cfg from impl crate

[JelteF]

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We can use rustversion for this. For some of the tests its still needed because of this error:

error[E0658]: non-inline modules in proc macro input are unstable
  --> tests/error/mod.rs:53:1
   |
53 | mod nightly;
   | ^^^^^^^^^^^^
   |
   = note: see issue #54727 <https://github.com/rust-lang/rust/issues/54727> for more information

[JelteF]

Hmm, okay apparently I cannot use rustversion in doc tests (which is where we need it). So I'll leave this open to look again later.